40 Mr. T. H. Blakesley on Magnetic Lag. 



force. Just as when a body moving in a medium does work 

 in the medium, it does so by calling into being, or inducing, 

 a force, viz. friction; force being that sort of magnitude which 

 acting upon a body produces motion. 



By analogy alone, therefore, we may infer that when 

 changing magnetization is a continuous source of absorbed 

 work, the changing magnetization induces what would itself 

 produce magnetization, that is an induced stress acting in 

 opposition to the direction of the change in the magnetization. 



This may perhaps be allowed; but it may be urged that 

 there is plenty of magnetic stress already, impressed by the 

 currents ; why should not the changing magnetization work 

 on this ? The answer to this objection is that if there be no 

 other stress but that impressed from outside, then the phase 

 of the magnetization will be in the same phase, and therefore 

 the increase of magnetization will be in quadrature with the 

 stress, and hence no work will be continuously absorbed. For 

 though through some phases work may be done in such a 

 case, this is always recoverable and recovered in a complete 

 period; a proposition which I have stated and proved in my 

 papers upon Alternating Currents in 1885. 



I apprehend, therefore, that besides the stresses AB BC 

 we have another induced stress in quadrature with the mag- 

 netization, because called into being by its increase, and 

 therefore in the same phase as FB or BC. 



Let BC, therefore, be produced until it meets in D the 

 line AD drawn at right angles to BC. 



Then AD will be the effective magnetic stress, i. e. that 

 magnetic stress which, maintained with a steady current, will 

 produce the actual magnetization; hence, if M is the maximum 

 magnetization, 



i^P = M, 

 P 



where p is called the magnetic resistance. Thus 



47T 



